Trimming apparatus for magnetic suspension systems

ABSTRACT

Apparatus is disclosed for trimming magnetic bearings and particularly for trimming magnetic bearings which employ two independently cooperating systems comprising a permanent magnet system and an electromagnet system. Adjustable permanent magnet means are disclosed for maintaining a suspended object in an equilibrium position which is invariant with applied force in at least one dimension. According to one embodiment the trimming is accomplished by mechanically moving adjustable permanent magnets. According to another embodiment of the invention the trimming is accomplished by varying the field of adjustable permanent magnets using at least one magnetizing winding.

United States Patent [191 Perper TRIMMING APPARATUS FOR MAGNETICSUSPENSION SYSTEMS [75] Inventor: Lloyd J. Perper, Tucson, Ariz.

[73] Assignee: Cambridge Thermionic Corporation, Cambridge, Mass.

[22] Filed: Aug. 6, 1971 [21] Appl. No.: 169,665

[52 us. Cl. 308/10 .[51] Int. Cl. Fl6c 39/06 [58] Field of Search308/10; 74/5 [56] 7 References Cited Y UNITED STATES PATENTS 3,490,8161/1'970 Lyman 302/10 3,243,238 3/1966 Lyman 308/10 2,436,939 3/1948Schug 308/10 2,508,444 4/1970 Sitomer 308/10 3,124,962 3/1964Hirtreiter. 308/10 PROCESSOR 1 IN 1 EUR/NOR SENSOR [451 I Feb. 12, 1974Primary Examiner-R. Skudy- Attorney, Agent, or Firm-Roger Norman Coe[57] ABSTRACT Apparatus is disclosed for trimming magnetic bearings andparticularly for trimming magnetic bearings which employ twoindependently cooperating systems comprising a permanent magnet systemand an electromagnet system. Adjustable permanent magnet means aredisclosed for maintaining a suspended object in an equilibrium positionwhich is invariant with applied force in at least one dimension.According'to one embodiment the trimming is accomplished by mechanicallymoving adjustable permanent magnets. According to another embodiment ofthe invention the trimming is accomplished by varying the field ofadjustable permanent magnets using at least one magnetizing winding.

7 Claims, 3 Drawing Figures PAIENTEI] FEB I 2 I974 SENSOR II I l I l.+

PROCESSOR INTEGRATOR PRIOR ART ATTORNEY.

PATENIEBFEBI 2mm 7 9 l ,7 O4

sum 2 OF 2 (DZ (Dz IIIIH PROCESSOR PROCESSOR I I |s INTEGRATOR SENSORSENSOR INTEGRATOR 25 '8 N 25 N N s S N V N V N s 1 /L S 3% s.

v INVENTOR.

Lloyd J. Perper IBY' {g ATTORNEY.

TRIMMING APPARATUS FOR MAGNETIC SUSPENSION SYSTEMS BACKGROUND OF THEINVENTION The invention described herein was made in the performance ofwork under a NASA contract and is subject to the provisions of Section305 of the National Aeronautics and Space 'Act of 1958, Public Law 85568 (72 Stat. 435;42 U.S.C. para2457).

The present invention relates to magnetic suspension systems, and moreparticularly, it is concerned with the utilization of adjustablepermanent magnets for trimming magnetic suspension systems.

Magnetic suspension of an object without fluid flotation means has beenadhieved using a pair of electromagnets exerting radial centering forcesand opposed axial pulling forces by variably energizing the magnetsaccording to the relative ratio of movement and displacement of theobject. An example of such a magnetic suspension system is described inLyman patent,

U. S. Letters Patent No. 3,473,852. In accordance with that system, amovable body is suspended by use of a pair of electromagnet forceappliers. These force appliers are controlled by stable amplifier meanswhich are mainly responsive to velocity of movement of the body andsecondarily responsive to the displacement of the body from apredetermined position between the force appliers. In said system, eachelectromagnet is composed of an armature and a re-entrant cup typestator which has a coil lining inserted therein. The inner and outerpole faces of the stator and armature are substantially coplanar and areconfigured with confronting circular projections or ridges to producesubstantial radial force components in response to, and for overcoming,slight departures from coaxial alignment between the electromagnets andtheir armatures.

The heretofore described-magnetic suspension systems have required thatthe electromagnets be continually activated in order to counteract allstatic and dynamic loads and thereby maintain the suspended body in adesired position. In order to effect the necessary control fairly largesteady state currents are required for the electromagnets. Because ofthe steady, state current requirements, multiple problems have arisen inattempting to obtain a self-contained magnetic suspension system,including thermal problems, size and weight problems, and power supplyproblems.

In 1842 Earnshaw developed a theorem-which states that it is impossibleto obtain stable magnetic suspension through the use of static fields.This incontestable law has led investigators to conclude that power isrequired for suspension of a body in a magnetic field. As disclosed inapplication Ser. No. 160,384 now abandoned, entitled VirtuallyZeroPowered Magnetic Suspension", filed July 7, 1971, Joseph Lyman hasdiscovcred that all static loads, including axial, radial and torsionalof a magnetic bearing can be carried by a field supplied throughpermanent magnets. This is achieved, without violation of Earnshawstheorem, through the use of auxiliary servo systems which function toprovide only active control by means of independent auxiliary fields.Accordingly, when a sustained external force, such as gravity or alinear acceleration, acts on an object magnetically suspended inaccordance with the invention of Lymans application, the electromagnetsystem functions to maintain the suspended object in the desiredposition and can also function to bring about a shift in equilibriumposition of the overall system.

SUMMARY OF THE INVENTION system are employed.

A further object of the present invention is to provide a system ofvirtually zero powered magnetic suspension with a zero centering error.

Still another object of the present invention is to provide a magneticsuspension system having an equilibrium position which will be invariantwith applied force in at least one dimension.

In accordance with the present invention, adjustable permanent magnetmeans are employed for trimming magnetic suspension systems therebyenabling a suspended body to be maintained in an equilibrium positionwhich is invariant with applied force in at least one dimension.According to one embodiment the trimming is accomplished by mechanicallymoving adjustable permanent magnets. According to another embodiment ofthe invention the trimming is accomplished by varying the field of theadjustable permanent magnets using a magnetizing winding.

BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, advantagesand features of the invention will be. apparent to those skilled in theart from the following detailed description thereof, taken inconjunction with the accompanying drawngs, in which:

FIG. 1 illustrates in a highly schematic form a prior art embodiment,with portions being shown in cross section;

FIG. 2 is a highly schematic drawing which illustrates an embodiment inaccordance with the present invention employing a mechanically operatedlead screw to regulate the position of permanent magnets, ith portionsof the drawing being shown in cross section; and

FIG. 3 is a highly schematic drawing of another embodiment of theinvention which illustrates the solid state implementation of theinvention for the avoidance of all mechanical friction in a magneticsuspension system, with portions of the drawing being shown in crosssection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG 1 illustrates the inventionset forth in Lyman application Ser. Nov 160,384, entitled Virtually ZeroPowered Magnetic Suspension, filed July seven, 1971. In this drawing arigid body 11, such as a cylinder, is suspended between electromagnets12 and 13 and permanent magnets 14 and 15. Permanent magnet assembliesl4 and 15 provide radial restoring forces, but contribute to axialinstability such that rigid body 11 is sensed by sensor 18, which may bea displacement and- /0r a rate sensor. As described in more detail in USLetters Patent No. 3,473,852, which is hereby incorporated by reference,sensor 18 can include any suitable means for sensing movement of thesuspended body 1 1, such as means for obtaining a rate component bymeans of one or more coils arranged to cooperate with a permanent magnetat the end of suspended body 1 1. Movement of the suspended body causeseither an increase or a decrease in the magnetic flux linkage with theturns of the coil thereby resulting in an output voltage of a polarityproportional to the rate of movement of the suspended body. Adisplacement component can be derived from a photoelectric arrangementhaving a light source, a shield and a photoelectric cell for sensing thedisplacement of suspended body 1 l. The output from sensor 18 isamplified and filtered in processor 20 and used to actuate force coils22 and 23 in electromagnets 12 and 13, respectively. Theseelectromagnets act to dynamically center suspended rigid body 11. Byintegrating the output from sensor 18 in integrator 25, the steady statedriving power applied to coils 22 and 23 of electromagnets 12 and 13 maybe gradually reduced. This is effected by displacing the equilibriumpoint toward the source of external force. The resultant axial force ofthe permanent magnets then balances the applied external force.

Considering FIG. 1 in more detail, suspended body I 11 is illustrated asthe figure of revolution about its longitudinal'axis, for example, acylinder. It may take the form of a gyroscope rotor, or may bevertically arranged as a rotary mass from which a centrifugal chamber isto depend. As with other magnetic suspension apparatus, brushes andmechanical bearings of conventional d-c motors, with their attendantproblems, are eliminated. For imparting high speed rotation to thesuspended body 11, a rotating induction field may be produced by motor16 which surrounds the suspended body. As mentioned above, thecontribution of permanent magnets 14 and 15 is unstable. As suspendedbody 11 moves toward permanent magnet 15, for example, the force orattraction of the permanent magnet increases and the force or attractionof the opposing permanent magnet 14 decreases. Without the electromagnetservo system, suspended body 11 would be a falling body, supportedradially by the fields of the permanent magnets. Stability, asdistinguished from the normal sustaining force, is provided by theelectromagnets l2 and 13.

Each of the electromagnets l2 and 13 comprises two cylindrical permeablemembers so constructed and arranged as to exert on suspended body 11 aforce parallel to the axis thereof when said body is aligned with theaxis of the electromagnets, and to exert a force component transverse tothe axis of suspended body 11 when it is slightly displaced from aposition of coincidence with its axis and with the axis of theelectromagnets. These electromagnets are energized by direct current,the intensity of energization being differentially controlled.

The outer cylindrical member of electromagnet 12 is composed ofpermeable material. The electromagnetic coil illustrated in the drawingis a stationary coil winding 22. This electromagnetic coil takes theform of a thin cylindrical shell in which overlapped windings arepresent in a suitable plastic matrix, such as epoxy. Electromagnet 13 issimilarly constructed with a cylindrical permeable member and a'stationary coil winding 23.

The electromagnets are used as part of the servo system to maintainsuspended body 11 in a zero center position, which is a position lyingon a plane cutting through the magnetic center of the field supplied bythe permanent magnets. At or in the region immediately adjacent to theaforementioned plane extremely small forces are required to maintain themagnetic center of gravity of the suspended body. The servo systemsenses may axial transitional velocity and acts to reduce this velocityand restore suspended body 11 to its original position.

Since the electromagnets can be regulated, the whole acceleration fieldcan be shifted axially. One manner of accomplishing this is by biasingan amplifier. Thus, if movable body 11 moves toward the lower permanentmagnet 15, and the pole'of that magnet is increased, the differentialupward pull of the upper electromagnet 12 is adjusted to increase muchmore steeply to restore stability of the overall system. Accordingly,the electromagnets can be so regulated to reduce the velocity of movablebody 11, stop it altogether and even reverse the velocity of said bodyuntil it is once again coincident with the neutral or zero accelerationposition. At the neutral or zero acceleration position, theelectromagnets are needed only to maintain the position since thepermanent magnets carry all static loads. Virtually zero power isrequired to maintain the movable body in the zero or neutralacceleration position once the movable body has been properlypositioned.

Without adjusting the accerlation field, differential upward pull (net)of the electromagnets minus the differential downward pull (net) of thepermanent magnets would be necessary to equal a steady state forcedownward (e.g., gravity). By adjusting the entire acceleration field,the electromagnets are relieved of the burden of producing a substantialsteady state net force. I

It will be seen that in FIG. 1, the equilibrium position of rigid body11 which issuspended magnetically is inhe'rently off center in thepresence of axialacceleration. This reduces the dynamic range ofmotion'in the direction of the displacement of rigid body 11 and causesunequal radial stiffness to be present at permanet magnet assemblies 14and 15. These problems are overcome in accordance with the presentinvention.

In FIG. 2, components which. are identical to that shown in FIG. 1 havebeen numbered identically..In

contrast to FIG. 1, integrator 25, instead of adding to the force coilcurrents, controls motor 28 which drives the lead screw 30, therebypositioning permanent magnet assembly 32, relative to disc 33 on theshaft of rigid body 11.

It will be seen that in accordance with the present invention anequilibrium position can be obtained which is invariant with appliedforce in at least one dimension. Motor 28 drives the permanent magnetassembly 32 to null the output of processor 20 and thereby obtain acondition such that the magnetic force produced by permanent magnetassembly 32 exactly balances out any applied external force withoutdisplacement of rigid body 11 which is suspended.

FIG. 3 illustrates an alternative embodiment to that shown in FIG. 2. Inaccordance with the embodiment shown in FIG. 3, the permanent magnetsystem comprising permanent magnets 35 and 36 are fixed physically, butare magnetically adjustable. By applying short, high energy pulses tomagnetizing coils 38 and 39 of magnetic material 35 and 36,respectively, the field of such magnetic material is adjustableincrementally. For example incremental changes can be obtained by ananalog method wherein the intensity of a magnetizing pulse transmittedto a magnetizing coil is controlled to produce a specified change in theoperating B-H characteristic of the magnetic material. According toanother method, the digital method, the magnetic material is separatedinto sections which are then driven to saturation in one direction orthe other according to a digital switching command system (not shown)generated by an analog-to-digital conversion of the integrated errorsignal from integrator 25. The choice of materials employed is obviouslyimportant. Ferromagnetic materials are suitable since these materialscanbe magnetized readily. Such materials include not only iron, nickeland cobalt but also alloys containing components which are notferromagnetic.

The embodiment of FIG. 3 overcomes certain difficulties with themechanical adjustment of the permanent magnet assembly 32 illustrated inFIG. 2. For example, in certain applications, such as utilization inspace, a problem occurs with mechanical bearings since the lubricantnecessary for such bearings tend to evaporate. The solid stateembodiment of FIG. 3 contains no moving parts in the control system.

From the foregoing, it will be seen that this invention is well adaptedto obtain all the ends and objects hereinabove set forth, together withother advantages which are obvious and which are inherent to the system.Although the present invention has been described in terms ofmaintaining an'equilibrium position in a magnetic suspension systememploying both permanent magnets and electromagnets, it will beunderstood that the present invention can be used in connection withother types of magnetic suspension systems, including magneticsuspension systems employing only electromagnets, such as thatillustrated by Lyman US. Pat. No. 3,473,852.

Regardless of the magnetic suspension system employed, the presentinvention can be used advantageously not only to maintain a suspendedobject in a desired position but can also be used advantageously forinitial axial centering of the suspended body. Under ordinaryconditions, the force coils of the electromagnets should be capable ofcentering a magnetically suspended object during-any start-up operation.By utilizing the present invention, start-up operations involvingmagnetic suspension systems are not as critically dependent upon theelectromagnets. For example, the mechanical advantage of lead screw 30(FIG. 2) can be employed for an initial centering operation to maintainthe suspended body 11 in a desired position until other servo systemsare engaged. In this manner, initial centering of suspended body can beobtained with reduced power supply energy storage requirements.

If desired, the system shown in the drawings may be horizontallydisposed. If it is operated in the earths gravitational field, thepermanent magnets 14 and 15 are then required to exhibit vertical forcecomponents. Actually, the improved apparatus of the present inventionpermits the apparatus to be used in a variety of different environments,with various amount of force due to gravity and/or vehicle acceleration.The magnetic suspension system will accommodate relative displacementsbetween the suspending portion and the suspending apparatus, includingangular as well as translation components.

It will be understood that the permanent magnets can be made from anysuitable material. However, samarium cobalt is a particularly suitablematerial where it is desirable to conserve space and minimize weight.

The present invention permits a physical object to be suspended in thepresence of varying disturbing forces solely through the inherent fieldsexisting in permanent magnets with the expenditure of extremely smalland almost nominal, amounts of power needed for control purposes andwithout necessitating adjustment of the acceleration field involved. Allstatic loads are carried by the permanent magnet'fields which restrainthe suspended object in five of the six degrees of freedom. For utility,only rotation about one axis is unrestricted.

The present invention, which provides an equilibrium position which willbe invariant with applied force in at least one dimension, is ofimportance in instruments such as gyroscopic devices, and is desirablein general to avoid reduction of dynamic range of control in thedirection of displacement. Using the present invention, the magneticsuspension system can be trimmed at equilibrium position so that minimumpower is absorbed by the force coils and other actuating means.Functional alternatives'are present which include solid state as well asmechanical implementation of the invention, thereby permitting the totalavoidance of mechanical friction in a magnetic suspension system.

Obviously, many other modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof.

What is claimed is: l. Trimming apparatus for magnetic suspensionsystems, comprising:

a movable object; mutually opposed first and second permanent magnetmeans for suspending said movable object; separate trimming permanentmagnet means for suspending said movable object; means for adjusting thefield of said trimming permanent magnet means to an equilibrium positioninvariant with applied force in at least one dimension; j first andsecond electromagnet force applying means for exerting electricallycontrollable additional forces on said movable object, said first andsecond electromagnet force applying means positioned to cause saidmovable object to return to its neutral force position when electricallycontrollable additional forces are exerted on said movable object;

and

means for differentially energizing said first and second electromagnetmeans to cause electrically controllable additional forces to act onsaid movable object.

2. Trimming apparatus as defined in claim 1 which also includes meansprimarily responsive to the compo.- nent of velocity of said movableobject in a direction parallel to its longitudinal axis for supplying aninput control signal to the means used for adjusting the field of saidtrimming permanent magnet means.

3. Trimming apparatus as defined in claim 1 which further includes meansprimarily responsive to the displacement of said movable object from apredetermined position along its longitudinal axis for supplying aninput control signal to the means used for adjusting the field of saidtrimming permanent magnet means.

4. Trimming apparatus as defined in claim 1 wherein the means foradjusting the field of a trimming permanent magnet means to anequilibrium position invariant 7 with applied force in at least onedimension comprises a magnetizing winding around said trimming permanentmagnet means for establishing the magnetic field of said trimmingpermanent magnet means and means for energizing said magnetizingwinding.

5. Trimming apparatus as defined in claim 4 wherein the means forenergizing the magnetizing winding is accomplished by short electricalpulses.

6. Trimming apparatus as defined in claim 4 wherein the trimmingpermanent magnet means consists of magnetic material separated intosections and the magnetizing winding applied to said trimming permanentmagnet means permits magnetization of the sections of the magneticmaterial to be varied.

7. Trimming apparatus for magnetic suspension systems, comprising:

movable object

1. Trimming apparatus for magnetic suspension systems, comprising: amovable object; mutually opposed first and second permanent magnet meansfor suspending said movable object; separate trimming permanent magnetmeans for suspending said movable object; means for adjusting the fieldof said trimming permanent magnet means to an equilibrium poSitioninvariant with applied force in at least one dimension; first and secondelectromagnet force applying means for exerting electricallycontrollable additional forces on said movable object, said first andsecond electromagnet force applying means positioned to cause saidmovable object to return to its neutral force position when electricallycontrollable additional forces are exerted on said movable object; andmeans for differentially energizing said first and second electromagnetmeans to cause electrically controllable additional forces to act onsaid movable object.
 2. Trimming apparatus as defined in claim 1 whichalso includes means primarily responsive to the component of velocity ofsaid movable object in a direction parallel to its longitudinal axis forsupplying an input control signal to the means used for adjusting thefield of said trimming permanent magnet means.
 3. Trimming apparatus asdefined in claim 1 which further includes means primarily responsive tothe displacement of said movable object from a predetermined positionalong its longitudinal axis for supplying an input control signal to themeans used for adjusting the field of said trimming permanent magnetmeans.
 4. Trimming apparatus as defined in claim 1 wherein the means foradjusting the field of a trimming permanent magnet means to anequilibrium position invariant with applied force in at least onedimension comprises a magnetizing winding around said trimming permanentmagnet means for establishing the magnetic field of said trimmingpermanent magnet means and means for energizing said magnetizingwinding.
 5. Trimming apparatus as defined in claim 4 wherein the meansfor energizing the magnetizing winding is accomplished by shortelectrical pulses.
 6. Trimming apparatus as defined in claim 4 whereinthe trimming permanent magnet means consists of magnetic materialseparated into sections and the magnetizing winding applied to saidtrimming permanent magnet means permits magnetization of the sections ofthe magnetic material to be varied.
 7. Trimming apparatus for magneticsuspension systems, comprising: a movable object; separate first andsecond permanent magnet means for suspending said movable object; meansfor physically moving the second permanent magnet means whereby thefield of the second permanent magnet means is adjusted thereby enablingthe movable object to be maintained in an equilibrium position which isinvariant with applied force in at least one dimension; electromagnetmeans for exerting electrically controllable forces on said movableobject; and means for energizing said electromagnet means to achievestability of positioning of the suspended movable object.